Numbers of light sources have been proposed as replacements for the ubiquitous incandescent lamp and prominent among these are phosphor-converted lamps employing LEDs as the primary light source, with the use of a phosphor as a secondary light source. Chief among the latter type of lamp are those using blue-emitting LEDs having a phosphor layer immediately above the LED. The visible blue and/or ultraviolet light emitted by the LED energizes the phosphor to emit a substantially yellow light, the blue and yellow radiation combining to make an acceptable white light.
The end result of this system provides an initial source of light that covers a small area but with a wide, angularly dispersed white light source. The product of the source area and the solid angle of its output is known as etendue and is a quantity that cannot be reduced without loss as the light makes its way through an optical system. Owing to the power dissipation limitations of individual LEDs, one must combine the outputs of multiple LEDs to produce a useful reflector lamp and this condition is certainly true with respect to PAR lamps. The combination of multiple light sources necessarily increases the etendue of the system. Because of practical limitations, the etendue of the combined system includes a large amount of non-light producing areas that exist between the multiple LEDs. The result is a light distribution that requires a large reflector or other optical element to produce a narrow-angle beam. Additionally, some type of homogenizer or diffuser is usually required to reduce the perception of the individual LED chips.
Another problem that exists with use of phosphor to provide a specific light output from LEDs resides in the fact that the phosphor layer generates heat because of the so-called stokes shift or the difference in energy between the blue excitation photons and the lower energy emission. This heat is undesirable when it is close to the LEDs, which are very intolerant of elevated temperatures.
It has been suggested that the problem caused by excessive heat can be mitigated by placing the phosphor in a hemispherical silicone rubber dome, which dome is placed over the LED. While this approach achieves some beneficial improvement, it does not reduce the etendue problem; it actually increases it.